1. Technical Field of the Invention
The present invention relates to a moving image coding apparatus, which carries out band compression of moving image data to record the band compressed moving image data, and a moving image decoding apparatus, which decodes band compressed image data to regenerate image data from the band compressed image information.
A moving image data signal such as an output signal of a television camera, may be transmitted in real time, or be recorded once on a recording medium such as a disc device, and the recorded data may be regenerated later to use the moving image data. In the above case, the moving image data may be recorded as is or after being band compressed, in the same way as the moving image data signal may be transmitted as is or after being band compressed. However, there are some requirements for recording moving image data after being band compressed, which are different from the requirements for transmitting moving image data after being band compressed. Therefore, it is generally impossible to apply a conventional band compression technique which has been used in data transmission, to band compression for recording moving image data.
As a recording apparatus for moving image data, a video tape recorder or a film on which moving image data is recorded in an analog form, or a laser disc apparatus in which moving image data is recorded in a digital form, are known. In the above recording apparatuses, conventionally, moving image data is recorded as is. However, since the moving image data signals are wide band signals, there is a limit for recording a large amount of moving image data. Therefore, a technique for effectively recording moving image data by utilizing a method for band compression as used in the conventional apparatuses for transmitting data, is required.
2. Description of the Related Art
FIG. 1 shows a data format in a typical band compression method which is used in a conventional transmission apparatus. In the drawing, F denotes a frame synchronization pattern, R denotes an initial value of data, and D.sub.2, D.sub.3, . . . each denote an interframe difference data. The data R which is a reference value for use in calculation of the difference, is written in the first frame, and difference data Di (i=2, 3, . . .), which is a difference between data before being coded and data in a preceding frame, is written in each of the second frame and the following frames. Therefore, in a receiver side, when the first frame is received, the data R is used as a decoded data F.sub.1 of the first frame; when the second frame is received, the received data is added to the preceding data R (=F.sub.1) to obtain F.sub.1 +D.sub.2 as a decoded data F.sub.2 of the second frame; when the third frame is received, data F.sub.2 +D.sub.3 is used as a decoded data F.sub.3 of the third frame; and similar operations are carried out for the following frames. Since only difference data is transmitted in the second and following frames to realize band compression, the amount of transmitted data is small. For example, in the case of transmission of moving image data in a video telephone, full data of a picture of a speaker is transmitted as the above data R of the first frame, and then, only difference data from the preceding frame is transmitted for each of the second and following frames until the transmission is completed. If an error occurs, regeneration of images of the frame in which the error occurs, and the following frames, is impossible. Therefore, full data of a whole picture, instead of difference data, is transmitted when an error occurs, or full data, instead of difference data, is included for a different pixel in each frame so that complete data of a picture can be regenerated after at least a predetermined time elapsed from the occurrence of an error.
In the recording of moving image data, a function of making a data base of the moving image data, and random-accessing a requested portion of the data base; and a function of scanning the moving image data in a forward or backward direction, and the like, are necessary. However, when the above-mentioned method of band compression which is used for transmitting data is applied to recording of moving image data, and the data as shown in FIG. 1 is recorded, it is impossible to regenerate an image when starting to read the recorded data from a frame following the first frame, or when skipping frames on the way in the forward scanning operation.